1. Field of the Invention
This invention relates to an insulated board used in a wiring board, especially preferably an insulated board used as a wiring board in a package, a dielectric resonator, an LC filter, a dielectric waveguide line, and a dielectric antenna, in which high frequency signals such as microwaves or millimetric waves are transmitted.
2. Description of the Prior Art
As multilayer wiring boards made of ceramics, wiring layers composed of high melting metals such as tungsten or molybdenum on the surfaces or the interior of an insulated substrate composed of alumina sintered body have been spread most.
With highly information-oriented society in recent years, frequency bands of signals used in various electrical appliances have become higher frequencies. A higher frequency wiring board used in transmitting high frequency signals is required that in transmitting high frequency signals without a loss, the resistance of the conductor forming a wiring layer should be small, and that the dielectric loss in the high frequency region of the insulated board should be small. However, since the high melting metals such as tungsten and molybdenum have a high conductor resistance, these metals cannot be applied to a wiring substrate used in transmitting signals in a millimetric wave region of at least 30 GHz. In such a wiring substrate, it is necessary to use low-resistance metals such as copper, silver and gold.
However, the wiring layer composed of such a low-resistance metal cannot be formed by co-firing with alumina constituting the insulated board. Accordingly, in recent years, an insulated board composed of glass or a composite material comprising glass and ceramics, namely an insulated board made of a glass-ceramic, has been developed.
Japanese Laid-Open Patent Publication No. 12639/1992 discloses a multilayer wiring substrate obtained by simultaneously firing a molded product composed of a mixture of glass and an SiO.sub.2 -type filler and a wiring layer composed of a low-resistance metal such as copper, silver and gold at 900 to 1000.degree. C. Japanese Laid-Open Patent Publication No. 240135/85 discloses a multilayer wiring substrate prepared by simultaneously firing a molded product composed of a mixture of a zinc borosilicate type glass and a filler such as Al.sub.2 O.sub.3, zirconia and mullite and a low-resistance metal. Japanese Laid-Open Patent Publication No. 298919/1993 discloses an insulated board formed from a glass-ceramic material in which mullite or cordierite is precipitated as a crystal phase.
The glass-ceramics insulated board disclosed in the above prior art can be formed by simultaneously firing together with a low-resistant metal such as copper, silver and gold. However, a dielectric loss is high, so it cannot be used in the wiring substrate to which a high frequency wave signal having a high frequency of at least 1 GHz is applied. The glass ceramic sintered body has a thermal expansion coefficient of about 3 to 6 ppm/.degree. C. which is lower than the alumina sintered body, and has a defect that the thermal expansion difference from an organic resin material of a printed substrate used in a mother board is large. When a wiring substrate having an insulated board composed of a glass ceramic sintered body is mounted on the mother board having the printed substrate, a thermal expansion difference occurring between the printed substrate and the insulated board due to the heating such as soldering at the time of mounting is large. The stress generated due to the thermal expansion difference causes cracks in the mounting portion, and is liable to cause the wiring layer to peel. Furthermore, when such a wiring board is used in a package in which a semiconductor element is accommodated, the same problem is raised due to a thermal stress caused at the time that the operation/stopping of the semiconductor are repeated.